Mercury is naturally found in coal in concentrations ranging from 20 to 1000 ppb, and coal-fired power plants account for about 30% of global anthropogenic mercury emissions. All forms of mercury present in the coal decompose during combustion into the highly volatile elemental form (Hg0), which can readily evade capture by existing air quality control devices typically found at power plants. However, if the mercury can subsequently be oxidized to an ionic form (Hg2+), the mercury is much more readily captured by fly ash and sorbents and/or scrubber liquors. As a result, it is advantageous to maximize the conversion of elemental mercury to the oxidized form to enhance capture.
The dominant technology in use today to effect conversion of elemental mercury to oxidized mercury is the use of bromine-containing additives such as calcium bromide (CaBr2), which has been shown to oxidize mercury much faster than the native chlorine present in many coals. However, the use of bromine additives results in formation of hydrobromic acid (HBr), which will either be emitted from the stack or become dissolved in scrubber liquors and waste waters, which is of concern for coal burning utilities that use wetlands to treat waste water. Further, the issue of whether the use of bromine additives will accelerate high-temperature corrosion mechanisms remains an open question.
Thus, there is a continuing need for methods of reducing the release of mercury into the environment, as well as preventing pollutant release resulting from mercury sequestration processes. More particularly, there is a need for techniques and products to oxidize gaseous elemental mercury without concomitant formation of harmful byproducts.